Recently, power transmission devices performing contactless power transmission have been widely used. The power transmission device includes a power transmitter for transmitting power, and a power receiver for receiving transmitted power. The power transmission device executes contactless power transmission from the power transmitter to the power receiver using electromagnetic induction system, magnetic field resonance system, electric field coupling system, or the like. The power receiver contains a driving circuit for driving itself, a charging circuit for charging a secondary battery mounted on the power receiver, and the like.
For contactless transmission of power (up to about several tens of watts) to a portable terminal, a notebook computer or other electronic devices, it is generally required to bring the power transmitter and the power receiver into very close proximity with each other within the power transmittable range when the electromagnetic induction system or the electric field coupling system is used. On the other hand, when the magnetic field resonance system is used, the requirement of very close proximity between the power transmitter and the power receiver is not as stringent. For example, power transmission is capable even when the power receiver is separated from the power transmitter by several centimeters. Accordingly, the magnetic field resonance system is attracting attention in view of the advantage provided by the system that the power receiver may be more freely positioned and therefore more easily handled.
The magnetic field resonance system can transmit power by using coupling between a resonance element composed of a coil and a capacitor provided on the power transmitter, and a resonance element composed of a coil and a capacitor provided on the power receiver. In the electromagnetic induction systems, an attempt to increase the power transmission distance is similarly made by providing a resonance capacitor on each of the power transmitting side and the power receiving side and allowing resonance coupling between the element on the power transmitting side and the element on the power receiving side as well as coupling between the coil on the power transmitting side and the coil on the power receiving side. Thus, the differences between the magnetic field resonance system and the electromagnetic induction system are becoming less noticeable.
Parameters affecting power transmission efficiency include a coupling coefficient k between the resonance elements of the power transmitter and the power receiver. When the distance between the resonance elements of the power transmitter and the power receiver changes, the coupling coefficient k generally varies in accordance with the change in the distance. For example, when the distance between the resonance elements increases, the coupling coefficient k becomes smaller. When the impedance of a circuit is constant, power transmission efficiency varies in accordance with the change in the coupling coefficient k.
There exists technology for maintaining high power transmission efficiency even when the coupling coefficient k varies in accordance with the change in the distance between the resonance elements of the power transmitter and the power receiver. Such technology employs an impedance adjuster that changes the impedance of the power transmitter and the power receiver according to variations in the coupling coefficient k.
According to this technology, however, an additional circuit for automatically controlling the impedance in response to changes in the coupling coefficient k is required. This causes a problem in that control becomes more complicated.